Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting, and a wider spectrum of colors of emitted light in full-color OLED displays.
Applications of OLED devices include active matrix image displays, passive matrix image displays, and area lighting devices such as, for example, selective desktop lighting devices. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device.
The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
In top-emitting OLED devices, light is emitted through an upper electrode or top electrode which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s) can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Consequently, the lower electrodes (anodes) can be formed over relatively complex drive circuitry in an active matrix OLED image display. Top-emitting OLED displays offer the potential to improve display performance compared with bottom-emitting OLED displays by:    1) increasing the aperture ratio, therefore permitting pixels of the display to operate at a lower current density which results in improved operational stability;    2) permitting more complex drive circuitry to enable improved control of pixel current, leading to enhanced display uniformity and to improved display stability;    3) enabling the use of lower mobility materials, e.g., amorphous silicon, to be considered in forming the thin-film transistor (TFT) drive circuitry; and    4) permitting incorporation of elements which can increase the out-coupling of light generated within the organic EL medium structure to provide increased efficiency of emitted light.
However, bottom-emitting OLED devices continue to find widespread use in displays of data or in the field of advertising.
Unprotected OLED display devices, irrespective of device configuration, are prone to relatively rapid degradation of performance due to adverse effects of moisture and/or oxygen present in the ambient environment. Additionally, unprotected devices can be subject to mechanical damage caused by abrasion. Various efforts have been directed at providing packaged OLED displays in which the packaging approaches offer improved operational lifetime of displays which is, however, still limited so that widespread adoption of OLED display devices is currently restricted.
FIG. 1 is a schematic sectional view of a conventionally packaged OLED device in which a transparent cover plate is sealed to a device substrate by a perimeter seal. Moisture-absorbing desiccant material is provided in portions of the spacing between an uppermost surface of the OLED device and a lower surface of the cover plate. Alternatively, U.S. Patent Application Publication 2002/0187775A1 by Maruyama et al. incorporates an inert gas in the space between a device substrate and a second substrate which functions as a cover plate.
A perimeter seal is proposed by Maruyama et al. which is formed between two concave grooves disposed near perimeter areas of an OLED device. Since these concave grooves accept overflow of perimeter seal material during the pressing of a grooved cover plate to the device substrate, they do not prevent flow of the perimeter seal material into the concave grooves. Maruyama et al. do not disclose any flow-preventing elements for flowable material residing over the display area of an OLED device.
Wei et al. in U.S. Patent Application Publication 2002/0193035A1 disclose a package method and apparatus for organic electroluminescent display. A certain amount of an ultraviolet curing resin or thermal curing resin is spread on a lamination plate or a substrate. A trench is formed at an edge of the lamination plate. Upon aligning the lamination plate with the substrate, the space between the lamination plate and the substrate is controlled by adjusting lamination pressure so that excess resin flows into the trench at the edge of the lamination plate, and the dimensions of the package can be controlled. The resin is cured by ultraviolet radiation or by a thermal process.
All trench configurations in Wei et al. at edges of the lamination plate are perimeter trench configurations which accept overflow of excess resin material. The trenches in Wei et al. do not prevent flow of the perimeter seal material into the concave grooves.
Park et al. in U.S. Patent Application Publication 2002/0155320 A1 disclose a package method and apparatus for organic electroluminescent display. A trench is disposed on at least one of the cover plate or device substrate to prevent perimeter sealing material from contacting the display area of the OLED device. During pressing of the cover plate to the substrate, excess perimeter sealing material resin flows into the trench, and the sealing material is prevented from contacting the display area.
While perimeter seals offer improved moisture protection, the lack of structural buffer layer between the OLED device surface and the lower surface of the cover plate can cause mechanical and optical problems. Mechanical problems include excessive stress to the perimeter seal caused by thermal expansion and contraction under normal device operating conditions leading to leakage of the perimeter seal. Expansion of the gas in the space between the OLED device surface and the lower surface of the cover plate can lead to breakage of the device substrate or cover plate when subjected to lowered environmental pressure, especially for larger-sized displays. Optical problems include undesirable reflective or refractive optical effects at both surfaces of a transparent cover plate which is used in a top-emitting OLED display device.
U.S. Pat. No. 6,268,695, assigned to Battelle Memorial Institute, describes an environmental barrier for an OLED in which a glass cover plate is not used. In this invention, the foundation and the cover plate are coated with three layers: a first polymer layer; a ceramic layer; and a second polymer layer. Either the ceramic layer(s) in the foundation, the cover plate, or both, are substantially transparent to the light emitted by the OLED. This invention creates an environmental barrier for an OLED display, but does not provide mechanical protection for the OLED display, especially from pressure points such as those created when a user touches the surface of the display with his or her finger.
Other effective barrier layers against moisture penetration and/or oxygen penetration into an OLED device include a transparent encapsulation layer which can be formed by known thin-film deposition methods such as, for example, thermal vapor deposition, sputter deposition, or atomic layer deposition. Materials particularly suitable as encapsulation layer materials include aluminum oxide (Al2Ox), silicon nitride (SiN), silicon-oxinitride (SiOxN1-x), and tantalum oxide (TaOx).
Due to the structure of the thin film encapsulation layers, they do not provide adequate mechanical protection. A transparent cover plate is required to ensure mechanical protection. However, conventional perimeter sealing of the cover plate to the OLED display substrate results in the aforementioned mechanical and optical problems.
In manufacturing OLED display devices, a plurality of devices are typically manufactured on a device substrate, and are subsequently singulated or cut and separated from the device substrate. Each OLED display device includes a pixelated display area and an electrical interconnect area which is used to connect the singulated OLED display device to external electrical power and control electronics.
Irrespective of the configuration of environmental protection elements, such as an encapsulation layer and a perimeter-sealed cover plate, or just a perimeter-sealed cover plate, it is important to keep at least the outermost portions of the electrical interconnect area free of encapsulation layer material and of perimeter seal material to ensure reliable electrical connections to the interconnect area or areas.
Thus, a need exists for a manufacturing method of providing an environmental barrier over a plurality of OLED display devices formed on a common device substrate, with the environmental barrier having improved mechanical properties and providing improved out-coupling of light emission in singulated OLED devices, and the environmental barrier being configured so that electrical interconnect areas of singulated OLED devices are readily accessible for making reliable electrical connections of electrical leads.